Integrated circuits are in such wide use now that the typical consumer relies on one or more of them several times each day, and takes for granted the sophisticated technology embodied in these circuits. The best known integrated circuits are incorporated into chips made of such materials as silicon or gallium arsenide and referred to by the industry as "dies," each die containing a multitude of circuit elements capable of performing complex electronic functions.
In use, these dies are incorporated into electronic systems which access the circuitry in the dies and to which the dies contribute their own specific functions. Incorporation into these systems is achieved in a variety of ways. Typically, the die is mounted to a substrate such as a printed wiring board, with wire leads joining the circuitry of the die to that of the board. The die and the wire leads joining the circuitry of the die to that of the board are protected either by a concave lid of either metal or plastic, bonded along its rim to areas of the board beyond the wire leads, or by embedding the die and wire leads in a shaped mound of molding compound molded in place by transfer molding or similar methods. In either case, the printed wiring board and protective cover form a housing for the die, and the combination of board, die and cover is termed a "package."
The use of molding compounds entails certain disadvantages over metal or plastic lids. One disadvantage is that molding compounds are generally heat insulators, causing the die to accumulate heat which can interfere with the functional performance of the package. Another disadvantage is the occurrence of frequency drifts in high frequency devices operating above one gigaherz, the drifts being caused by contact of the die with the molding compound.
With either type of protective covering, the assembled package has additional leads and contact areas on the periphery or the underside of the board which remain available for connections to external electronic circuits. Pin grid array packages are those with exposed pins to form the connections, and ball grid array packages are those with solder balls for the same purpose. The use of pin grid array packages with external circuity is often referred to as "plug-in" technology.
A concern in the fabrication of integrated circuit packages is the heat generated by the current passing through the circuit, and the accompanying risk of burnout of the die. To prevent burnout, many packages in current use are limited to 2 watts. For dies designed to carry higher currents, however, the heat generation requires a heat sink or other means of dissipating the heat to avoid burnout. Conventional packages themselves cannot dissipate high amounts of heat, since printed wiring boards are typically fabricated of ceramic for the high reliability needed in military and industrial applications, or of plastic for low-cost commercial and consumer products, and both of these materials are thermal insulators.
High current density with effective heat dissipation has been achieved in plug-in packages which have substrates made of alumina ceramic, since alumina ceramic is a good heat conductor and the heat dissipation is augmented by the metal pins. Packages of this type are relatively costly, however.
To increase heat dissipation, special structures for both the substrate board and the cover have been devised. Metal sheets, for example, have been laminated into board structures to serve as heat spreaders. This requires a specially constructed board, however, both to incorporate the metal sheet and to avoid interference of the sheet with the circuitry in the board. In molded packages, heat-dissipating fin structures have been bonded to external surfaces of the packages. This unfortunately increases the bulk of the package and still relies on transmission of the heat through the molding compound to the fin structure.